Effluent treatment with hydrotalcite

ABSTRACT

Effluents containing residual dyes are contacted with an effective amount of hydrotalcite-like material together with an effective amount of a magnesium salt or a mixture of magnesium salts, the weight ratio (on dry basis) of hydrotalcite-like material to magnesium salt being in the range of 20:1 to 1:20. Residual dyes are removed from effluents.

This application claims benefit of international application PCT/EP94/04177, filed Dec. 15, 1994.

FIELD OF THE INVENTION

The present invention relates to effluent treatment. The presentinvention more specifically relates to a new adsorbent and its use in aprocess for removing residual dyes from effluents.

BACKGROUND OF THE INVENTION

The colour-using industries such as textiles industry, paper industryand leather industry produce effluent containing residual dyes whichneed to be removed.

It is already known from European Patent Application 91911139.3 thathydrotalcite like materials can be used to adsorb colours to lowresidual levels.

Any effluent treatment is characterized by the treatment of largevolumes of effluent. In order to be cost effective when treating largevolumes of effluents, there is a need for a process wherein additionlevels of adsorbents are minimized.

Therefore there is a need for a process which enables a quick removal ofresidual dyes from effluents, in order to meet consent limits and insome instances to permit partial or total water reuse, while minimizingthe use of adsorbents.

It has now been found that by adjusting the pH it is possible todramatically increase the capacity of the adsorbent to adsorb dyes.

It has now been also found that by adding a magnesium salt tohydrotalcite-like materials, a synergistic effect is obtained whichenables hydrotalcite like materials to be used much more effectivelythan before.

DEFINITIONS AND TESTS

i) Hydrotalcite like material

By hydrotalcite-like materials it is understood products which can bedescribed by the formula below:

    M.sub.k+m N.sub.n+p (OH).sub.2 A.sub.z.sup.y- x.H.sub.2 O

where:

M is any 1+ or 2+ cation or combination thereof

N is any 3+ or 4+ cation or combination thereof

k is the sum of the individual mole fractions of the 1+ cations

m is the sum of the individual mole fractions of the 2+ cations

n is the sum of the individual mole fractions of the 3+ cations

p is the sum of the individual mole fractions of the 4+ cations

where either but not both of k and m or n and p can be zero andk+m+n+p=1.

A_(z) ^(y-) is any anion of charge y- and mole fraction z, orcombination of anions of similar or differing y- and k+2m+3n+4p-2-zy=0and x can range from 1 to 100.

Examples of the cations M in the above formula are Li, Mg²⁺, Zn²⁺, Fe²⁺,Cu²⁺, Sn²⁺, Ca²⁺ and Sr²⁺. Suitable N cations include Al³⁺, Fe³⁺, Ti⁴⁺and Sn⁴⁺.

Preferred divalent cations are Zn²⁺, Cu²⁺ or Mg²⁺ or a combination ofthese ions, or a combination with other divalent cations.

The anion A may be an inorganic or organic anion. Preferred inorganicanions A are Cl⁻, NO₃ ⁻, SO₄ ²⁻, CO₃ ²⁻ and OH⁻. Examples of organicanions are carboxylate ions such as citrate and stearate.

Examples of hydrotalcite-like materials of this invention are:

Zn16 Al2 (OH)36 (Cl2) xH2O

Mg6 Al2 (OH)16 (NO3)2 xH2O

Cu16 Al2 (OH)36 (Cl2) xH2O

Preparation of hydrotalcite-like compounds is known and has beendescribed in a number of publications including Solid State Ionics 22(1986) pages 135-141 where there is published a paper by Walter TReichle entitled "Synthesis of Anionic Clay Minerals (Mixed MetalHydroxides, Hydrotalcite)" and Chemtech (January 1986) pages 58-63 wherethere is published a paper by Walter T Reichle entitled "Anionic ClayMinerals".

ii) Capacity

To measure the capacity of adsorbent to adsorb dyes the light absorbance(A0), before treatment, and the light absorbance (A1), after treatmentare measured and the capacity is defined by

    Capacity=100*(1-A1/A0)

GENERAL DESCRIPTION OF THE INVENTION

It is a first object of the present invention to provide a process forremoving residual dyes from effluents wherein the pH of the effluent tobe treated is first lowered to between pH 4 and pH 5.5, an effectiveamount of hydrotalcite-like material being then added the effluent to betreated, the pH being then raised to above 7.

By first decreasing the pH and then creating a caustic chock, thecapacity of the adsorbent to adsorb dyes has been found to be improved.

Preferably the hydrotalcite like material is a product which can bedescribed by the formula below:

    M.sub.k+m N.sub.n+p (OH).sub.2 A.sub.z.sup.y- x.H.sub.2 O

wherein M is Mg and N is Al (Mg/Al hydrotalcite).

It has also been found that the addition of a zinc salt or a magnesiumsalt to the hydrotalcite like material after decreasing the pH andbefore raising it again gives rise to a further increase of the dyeadsorption.

It is second object of the present invention to provide a new adsorbentcontaining:

5% to 95% by weight (on dry basis) of hydrotalcite like material,

5% to 95% by weight (on dry basis) of metal salts selected from thegroup comprising magnesium salts, zinc salts and mixtures thereof.

Preferably, the (Hydrotalcite/Magnesium salt) weight ratio (on drybasis) is with in the range 20:1 to 1:20, more preferably within therange 10:1 to 1:10, even more preferably 3:1 to 1:3.

Magnesium salts such as MgCl₂ and MgSO₄ have been found to beparticularly useful.

SPECIFIC DESCRIPTION OF THE INVENTION

The invention will be further described in the following examples withreferences to FIGS. 1 to 3 which represent UV visible absorbance scansshowing the dye content of the aqueous phase before and after treatmentwith an adsorbent.

EXAMPLE 1

A stock dye solution was made up by adding 0.2 g of the following dyesto 1 liter of water.

Tetramine Blue FGC

Cibacron Scarlet F-3G

Procion Turquoise HA

Procion Red HE3B

Sumifix Supra Blue BRF

Remazol Turquoise Blue G133

Levafix Brilliant Blue E8

Direct Red 80

The pH of the stock dye solution was raised to 12 by addition of causticsoda, and was stirred for 30 mins at 90° C. This was to ensurehydrolysis of the dyes and hence more closely mimic a real effluent. Thedye solution that was treated was made up by adding 50 ml of theconcentrated dye solution to 950 ml of water.

Three treatments were carried out using the same method in each case.

Method

1. Take 11 of diluted dye solution.

2. Lower the pH to 5.0 by addition of HCl.

3. Add adsorbent.

4. Stir at 30° C. for 20 mins.

5. Raise pH to 10.0 by addition of caustic soda.

6. Add 1 ml of a 0.1% solution of Allied Colloids Magnafloc 1011 (whichis a coagulant acting as a separation aid).

7. Stir for 1 min then allow to settle.

8. Filter a 10 ml sample of the resulting supernatant through a 0.45 μmcellulose nitrate filter.

9. Measure the absorbance from 400-700 nm on a Perkin-Elmerspectrometer.

The three treatments differed in the adsorbent system added at stage 3of the method which was varied as follows:

Treatment 1. add 0.1 g of hydrotalcite-like material (obtainable fromJoseph Crosfield & Sons, Warrington, U.K., under the trade nameMacrosorb CT100).

2. add 0.5 g Magnesium Chloride Hexahydrate.

3. add 0.05 g Macrosorb CT100 (obtainable from Joseph Crosfield & Sons,Warrington, U.K., under the trade name Macrosorb CT100) and 0.25 gMagnesium Chloride Hexahydrate.

It was found that treatment 3 was better than either 1 or 2 showing asynergy when using half dosages of Macrosorb/Magnesium Chloride ratherthan a full dose of each independently.

FIG. 1 proves the synergistic effect between magnesium chloride andhydrotalcite-like material.

EXAMPLE 2

In this example the influence of the hydrotalcite/Mg Cl₂ ratio wasassessed by repeating the procedure disclosed in Example 1, the totalweight of hydrotalcite and MgCl₂ being kept constant and the capacitybeing measured.

The results are summarised in the following table.

    ______________________________________                                        Hydrotalcite   MgCl.sub.2, 6H.sub.2 O                                         (parts by weight)                                                                            (parts by weight)                                                                            Capacity                                        ______________________________________                                        10             90             17                                              25             75             19                                              40             60             22                                              50             50             23                                              75             25             25                                              ______________________________________                                    

EXAMPLE 3

The procedure in Example 1 was repeated except that the pH was decreasedto 4 instead of 5. The capacity was found to increase from 25 to 30.

EXAMPLE 4

Completely dissolving the adsorbent at pH 2 and then raising the pH backto 10 gave rise to a complete removal of the dyes, the capacity in thiscase being almost 100.

EXAMPLE 5

A stock dye solution was made up as in Example 1 except that 0.4 g ofeach of the following dyes was added to 1 L of water.

Tetramine Blue FGC

Cibacron Scarlet F-3G

Remazol Turquoise Blue G133

Levafix Brilliant Blue E-8

Ramazol Red RB

The method was the same as in Example 1 except that the following wereadded at stage 3 of the method.

Treatment 1. 0.2 g of hydrotalcite-like material (obtainable from JosephCrosfield & Sons, Warrington, U.K., under the trade name MacrosorbCT2000)

2. 0.2 g Magnesium chloride hexahydrate

3. 0.1 g of hydrotalcite-like material (obtainable from Joseph Crosfield& Sons, Warrington, U.K., under the trade name Macrosorb CT2000M), and0.1 g Magnesium chloride

4. 0.2 g Aluminium sulphate hexadecahydrate

5. 0.1 g of hydrotalcite-like material (obtainable from Joseph Crosfield& Sons, Warrington, U.K., under the trade name Macrosorb CT2000M), and0.1 g Aluminium sulphate

6. 0.2 g Ferrous sulphate heptahydrate

7. 0.1 g of hydrotalcite-like material (obtainable from Joseph Crosfield& Sons, Warrington, U.K., under the trade name Macrosorb CT2000M), and0.1 g Ferrous sulphate

8. 0.2 g Ferric sulphate hexahydrate

9. 0.1 g of hydrotalcite-like material (obtainable from Joseph Crosfield& Sons, Warrington, U.K., under the trade name Macrosorb CT2000M), and0.1 g Ferric sulphate

10. 0.2 g Calcium chloride

11. 0.1 g of hydrotalcite-like material (obtainable from JosephCrosfield & Sons, Warrington, U.K., under the trade name MacrosorbCT2000M), and 0.1 g Calcium chloride

12. 0.2 g Zinc Chloride Hexahydrate

13. 0.1 g of hydrotalcite-like material (obtainable from JosephCrosfield & Sons, Warrington, U.K., under the trade name MacrosorbCT2000M), and 0.1 g Zinc Chloride

Only in the case of Magnesium chloride was there found to be a synergyat all wavelengths from 400-700 nm, this synergistic effect beingdisclosed on FIGS. 2.1 and 2.2. Zinc Aluminium and Iron (III) showsynergy above approximately 570 nm. Iron (II) and Calcium show nosynergy.

EXAMPLE 6

The general procedure disclosed in Example 1 was repeated except the pHafter the addition of the adsorbent was only raised to pH 7.5.

In a first test the adsorbent was 0.05 g of Hydrotalcite like material(CT100) and 0.25 g of MgCl₂. The capacity was found to be 24.

In a second test the adsorbent was 0.05 g of Hydrotalcite like material(CT100) and 0.25 g of ZnCl₂. The capacity was found to be 45.

Therefore using ZnCl₂ instead of MgCl₂, gives rise to a dramaticincrease in capacity together with the added benefit that lest causticis necessary since the pH is only raised up to 7.5. The difference withExample 5, test 13 is that, in the former case the pH was raised up to10, leading to a complete dissolution of ZnCl₂. It is the reason why,when using ZnCl₂, the pH has to be kept between 7.5 and 8.5.

EXAMPLE 7

The dye solution and method were the same as in Example 5, except thefollowing were added at stage 3 of the method.

Treatment 1. 0.1 g of hydrotalcite-like material (obtainable from JosephCrosfield & Sons, Warrington, U.K., under the trade name MacrosorbCT2000M), and 0.1 g Magnesium sulphate (ratio=1:1).

2. 0.05 g of hydrotalcite-like material (obtainable from JosephCrosfield & Sons, Warrington, U.K., under the trade name MacrosorbCT2000M) and 0.15 g Magnesium sulphate (ratio=1:3).

3. 0.15 g of hydrotalcite-like material (obtainable from JosephCrosfield & Sons, Warrington, U.K., under the trade name MacrosorbCT2000M), and 0.05 g Magnesium sulphate ratio=3:1.

Treatment 1 shows that there is a synergy with magnesium sulphate,proving therefore that the important element of the invention is the useof a magnesium salt.

Treatments 2 and 3 show that the synergy is also seen for other ratiosof hydrotalcite-like material to magnesium salt.

The results are summarized on FIG. 3.

We claim:
 1. Process for removing residual dyes from effluents whichcomprises lowering the pH of the effluent to be treated to between pH 4and pH 5.5, adding an effective amount of hydrotalcite-like material tothe effluent to be treated, and then raising the pH of the effluent toabove
 7. 2. Process according to claim 1 which comprises adding metalsalts selected from the group consisting of magnesium salts, zinc saltsand mixtures thereof to the effluent to be treated after havingdecreased the pH and before having raised it.
 3. Process according toclaim 2 which comprises adding ZnCl₂ to the effluent and then raisingthe pH to between pH 7.5 and pH 8.5.
 4. Process according to claim 2which comprises adding magnesium salt and then raising the pH to a pH ofat least
 9. 5. Process according to claim 4 wherein the weight ratio (ondry basis) of hydrotalcite-like material to magnesium salt is in therange of 20:1 to 1:20.
 6. The process of claim 1 wherein thehydrotalcite-like material added to the effluent is in the form of anadsorbent comprising:5% to 95% by weight (on dry basis) of thehydrotalcite like material, and 5% to 95% by weight (on dry basis) ofmetal salts selected from the group consisting of magnesium salts, zincsalts and mixtures thereof.
 7. The process of claim 6 wherein themagnesium salt is chosen from the group consisting of MgCl₂ and MgSO₄.8. The process of claim 6 wherein the zinc salt is ZnCl₂.